Reference values of pulmonary diffusing capacity during exercise by a rebreathing technique

Normal reference values of the pulmonary diffusing capacity for carbon monoxide (DL(CO)), diffusing capacity of the membrane (DM(CO)), and pulmonary capillary blood volume (VC) were derived by a rebreathing technique in 44 normal, healthy, nonsmoking individuals ranging from 17 to 68 yr of age. Simultaneous measurements of DL(CO), lung volumes, pulmonary blood flow, and septal tissue volume were determined at rest and during steady-state exercise of 25, 50, and 80% of maximal workload and at two levels of alveolar O₂ tension. DM(CO) and VC were calculated using the Roughton-Forster relationship. Using stepwise regression analysis, DL(CO) and DM(CO) (in ml CO/min/mm Hg) were found to be significantly related to pulmonary blood flow (Q̇C in L/min), age (in years), and body surface area (BSA in square meters). VC (in ml) was found to be significantly related to Q̇C and BSA. Normalization of measured date with respect to these variables largely eliminated male versus female differences. We conclude that Q̇C is the most important determinant of the recruitment of diffusing capacity. From rest to near-maximal exercise, DL(CO), DM(CO), and VC increase linearly with respect to Q̇C without evidence of reaching a plateau.